When looking for signatures of life outside of the Solar System, we're hampered by a number of problems. Earth is the only place in the Universe where we know life exists, so our current best hope of success lies in looking for Earth-like worlds. However, Earth is a relatively small planet, and it's inherently difficult to locate similarly sized worlds orbiting at ideal distances from their host stars. Even if we find such a planet, we have an additional challenge: determining its atmospheric composition to see if its compatible with life as we know it.

One possible resolution: look for exoplanets orbiting white dwarfs, the remnants of stars like our Sun. The advantages of these systems would be manifold: a white dwarf is much smaller than a star, so if a planet passes between it and us, far more light is blocked. And Avi Loeb and Dan Maoz proposed that at least some signs of life might have survived the deaths of these stars. The light emitted by the white dwarf could highlight any oxygen in the exoplanet's atmosphere, which would be seen as a strong hint of life.

Individual living organisms, or even massive populations like forests and cities, will remain too small to see for the foreseeable future. However, life as we know it leaves chemical evidence. The signatures of some of these chemicals, like chlorophyll (used by plants and some bacteria in photosynthesis), are obvious; others, like methane, can be either biological or non-biological in origin.

Molecular oxygen (O2) is an especially interesting marker of life. The oxygen in Earth's atmosphere was produced by living creatures such as cyanobacteria, which changed the chemical composition of air over billions of years through photosynthesis. Since no other world in the Solar System has a significant amount of molecular oxygen, finding it on another world would be a strong hint of life, present or past. (While oxygen atoms were forged by nuclear fusion, many of those atoms are bound up in molecules with other elements. Thus, carbon dioxide (CO2) is not necessarily a biomarker, while O2 likely is.)

Measuring the chemical composition of an exoplanet's atmosphere requires a bit of serendipity, though. When a planet transits its star, it crosses between the star and us, so that some of the star's light is blocked. If the planet has an atmosphere, some of the light may filter through the outer layers of the air during the transit. Then, if we're lucky, we might be able to identify some of the atmosphere's chemical components based on the wavelengths of light they absorb.

To date, only a few exoplanet atmospheres have been detected, though their full molecular makeup is unknown.

As Loeb and Maoz pointed out, that's a hard problem for stars like the Sun, or even the smaller fainter stars known as M dwarfs. The detections might require hundreds of hours of observation time over many successive transits, and even then would achieve success only if the exoplanet is sufficiently illuminated. Their solution: look at white dwarfs instead.

White dwarfs are the remnants of the cores of stars like our Sun. As such, they are very hot, but relatively small: one with a mass equal to the Sun is roughly the size of Earth. If a life-bearing planet survived the death of its star, then it might be detectable as it transits the white dwarf. The star's small size and hot surface would be advantageous: a transit would block proportionally more of the white dwarf's light, creating a far stronger signature than for any ordinary star. The authors proposed that small telescopes could monitor known white dwarfs for transits, even though those same instruments might be too small for hunting exoplanets orbiting normal stars.

If the exoplanet has molecular oxygen in its atmosphere, the white dwarf's light would exhibit a strong absorption line around 760 nanometers of wavelength, perfect for infrared observations using the upcoming James Webb Space Telescope (JWST).

However, as the authors pointed out, looking for water (an essential ingredient for life as we know it) is a harder problem. We don't currently know how much absorption to expect from water vapor on Earth-like planets in any environment, much less those orbiting white dwarfs. Molecular oxygen and carbon dioxide would be within the realm of possibility, though; Loeb and Maoz estimated these measurements would require 10 percent of the observing time necessary for exoplanets orbiting ordinary stars.

No planet has yet been found orbiting white dwarfs yet. However, the first exoplanets were discovered orbiting neutron stars, which are also stellar remnants. Also, the host stars for many of the exoplanets we have spotted will eventually become white dwarfs, and it's likely at least some of those planets could survive the stars' death, so there's nothing obvious to prevent their discovery.

White dwarfs are relatively numerous, and the authors of the present study suggested that future planet-hunting missions such as Gaia or Darwin could target them in hopes of identifying exoplanet candidates. In the end, a dead star may provide the best hope for finding life.

When looking for the signatures of life outside the Solar System, we're hampered by a number of problems. Earth is the only place in the Universe where we know life exists, so our current best hope of success lies in looking for Earthlike worlds.

Another limitation is that life as we know it here on our planet is the only kind of life we know how to look for. We have no understanding of what life would be like if it evolved under a completely different set of conditions.

There's a group of biologists who are trying to find alien life on our own planet. They are operating on the hypothesis that life had multiple starts on our planet and ours is just the one that became dominant. If we aren't the first they may have been others that survived and evolved along with us but re so different from us that life as we know it is not in direct competition with it in any way. so it's possible that we have alien micro organisms living in our own bodies but we have no way of detecting them.

hmm.. I thought most hints of life on earth will be destroyed when the Sun becomes a Red Giant... before it becomes a white dwarf....for instance water will evaporate, the atmosphere itself will be destroyed and most of earth would melt... when the Sun becomes a Red Giant. At least that was my understanding.

If that is that case, wouldn't the same happen to the Earth-like exo-planet orbiting a white dwarf? Would there be any useful chemical compounds left over...

Either way, any advances made in detecting exo planets is fascinating stuff.

hmm.. I thought most hints of life on earth will be destroyed when the Sun becomes a Red Giant... before it becomes a white dwarf....for instance water will evaporate, the atmosphere itself will be destroyed and most of earth would melt... when the Sun becomes a Red Giant. At least that was my understanding.

If that is that case, wouldn't the same happen to the Earth-like exo-planet orbiting a white dwarf? Would there be any useful chemical compounds left over...

Either way, any advances made in detecting exo planets is fascinating stuff.

jes

Pretty sure there's also been studies that show a strong possibility that the Earth itself will spiral into the the Sun and be engulfed.

I seem to remember Hat Monster covering the Sol->red giant scenario in an observatory thread, but I can't seem to find it right now. But IIRC, as the sun loses mass, the Earth's orbit will widen, so it wouldn't be engulfed by the (now red giant-sized) sun.

I'd be astounded if any life survived the red giant phase of a sun/earth like planetary scenario. Would be like looking for signs of life at ground zero of a nuclear detonation immediately after. Still a needle in a haystack type search. But who knows, worth a look.

When they make the movie, we will send an exploratory mission to this dead world, the first hint of life we have found outside our planet. Then the menace that caused their star to explode will follow us home.

"Late at night, a police officer finds a drunk man crawling around on his hands and knees under a streetlight. The drunk man tells the officer he’s looking for his wallet. When the officer asks if he’s sure this is where he dropped the wallet, the man replies that he thinks he more likely dropped it across the street. "Then why are you looking over here?", the befuddled officer asks. Because the light’s better here, explains the drunk man."

Well, I guess one scenario would be an advanced civilization moving their planet out to an acceptable orbit the more swollen their host star gets and then, should they survive the shedding of material during the transition to the white dwarf phase, they could conceivable move it back to a close orbit to maintain proper heat and light output. Of course that's a lof of assumptions and by that time they'd probably have left their system but who knows, the universe is a strange place.

Well, I guess one scenario would be an advanced civilization moving their planet out to an acceptable orbit the more swollen their host star gets and then, should they survive the shedding of material during the transition to the white dwarf phase, they could conceivable move it back to a close orbit to maintain proper heat and light output. Of course that's a lof of assumptions and by that time they'd probably have left their system but who knows, the universe is a strange place.

With the technology levels you assume I would think it'd be far easier (and cheaper) just to terraform some other planet and move the population there rather than actually significantly shifting the orbit of a planet.

If our Sun were to die and become a white dwarf, wouldn't the Earth be destroyed in the process? Seems looking for similar sized planets under these circumstances would be easier but pointless.

What the article didn't mention is that the people involved in the study are looking for systems with a white dwarf where the planet was caught around the white dwarf by either getting there after the death of the star, or being formed after the red giant phase. The process would be long enough (billions of years) for new life to evolve on the planet orbiting the white dwarf.

This is probably not the most common type of planetary system, but there are enough candidates out there to make it possible for us to spot such systems.

I found this neat little article that talks about white dwarfs and habitability. I always assumed that white dwarves would emit lots of X-Rays or some other kind of dangerous radiation that would prohibit Earth-like planets.

"There are two ways for such a [habitable zone] planet to get there. First, it might have been engulfed by the star and inspiraled. However, on its way to the habitable zone the planet would have been exposed to temperatures of as much as a million Kelvin, charring it to a cinder if not destroying it. ...

Second, the planet might have escaped engulfment and been scattered closer to the white dwarf due to interactions with a massive companion. This scattering would give the planet a very high-eccentricity (very elliptical) orbit such that it would reach quite close to the star, and then tidal friction could damp it into a circular, habitable-zone orbit. However in this case, a lot of orbital energy would have to be dissipated in the tidal fricative process. Temperatures on the planet would be on the order of thousands of Kelvin. Again, only a charred cinder would be left. Earth-sized planets in the habitable zones of white dwarfs are unlikely to be habitable."

So potential habitable planets will mostly depend on planet reformation out of debris disks.

Annoying, since once again our methods are set to explore the less interesting planets first (fewer systems), akin to early hot jupiter surveys.

Also to keep in mind is that double star systems will have problems. It seems to me the double-degenerate model has evolved into predicting the major observed properties of supernova type-1a (but I don't know much about supernovas) [ http://astrobites.org/2013/02/19/explod ... n-the-sky/ ]:

"A double white dwarf explosion addresses the problems of the white dwarf/red giant model: double-degenerate pairs are both common and would leave no trace (when one white dwarf explodes, so would the other). Pakmor et al. suggest a new explosion mechanism for the white dwarf/white dwarf pair–one that could lead towards a single model for all Type Ia supernovae.

In the Pakmor et al. model, a carbon and oxygen white dwarf grabs helium from a second, lower mass white dwarf, which can either be composed of carbon and oxygen, or of pure helium (Figure 2). ...

This theory nicely explains the variety of Type Ia supernova we see. Carbon and oxygen companions produce brighter supernovae that last for a long time since they have more material to fuse; helium companions produce dimmer supernovae that fade away quickly. In an extreme white dwarf/white dwarf interaction, the companion could be ripped apart by tidal forces and form a disk of material around the primary; this might explain rare, ultraluminous supernovae."

When searching for life around a White Dwarf I think we're missing a rather important step in the process of that star's configuration:

It blew up! Likely taking any planets near enough to support life (or at the very least their atmosphere) with it. Red Giants are a nasty transition, and the aftermath is...well, its an expansion or explosion on a stellar scale.

So, from what I read of this, we've got some very smart people looking for life in a solar systems whose sun has--by its nature--destroyed any life that would have existed close enough to benefit from what heat it supplies.

According to the first comprehensive phylogenetic work on cyanobacteria, it was the origin of this multicellular diversification that ushered in the atmosphere oxygenation. ["Evolution of multicellularity coincided with increased diversification of cyanobacteria and the Great Oxidation Event" [GOE], Schirrmeister et al, PNAS 2012.]

The emergence of complex multicellular life (eukaryote multicellulars) is a complex issue. But it may be that the oceans were never a stable enough environment for evolving such life until the sequestration of nitrogen had matured:

"In contrast to modern oceans, data from ancient rocks indicates that the deep oceans of the early Earth contained little oxygen, and flipped between an iron-rich state and a toxic hydrogen-sulphide-rich state. The latter toxic sulphidic state is caused by bacteria that survive in low oxygen and low nitrate conditions. The study shows how bacteria using nitrate in their metabolism would have displaced the less energetically efficient bacteria that produce sulphide – meaning that the presence of nitrate in the oceans prevented build-up of the toxic sulphidic state."

Cyanobacteria is the first modern nitrogen fixating organism as far as I know, evolving its terminal differentiation forms (so nitrogen fixation specialized cells) after the GOE. Among its effective nitrogen fixation products is nitrate:

"Heterocyst-forming species are specialized for nitrogen fixation and are able to fix nitrogen gas into ammonia (NH3), nitrites (NO2-) or nitrates (NO3-) ..."

[It may be that also carbon (carbon dioxide, methane) had to be sequestrated to avoid dips in oxygen levels.

Here it wasn't cyanobacteria, but instead possibly the first hardbodied multicellular eukaryotes, calciferous sponges, that captured and mineralized enough carbon inside their tough to eat skeletons that larger multicellulars eventually could stabilize the carbon cycle.

But again, small multicellulars may have plowed the way for large, and the stable oxygen based biosphere.]

Quote:

Since no other world in the Solar System has a significant amount of molecular oxygen, finding it on another world would be a strong hint of life, present or past.

That is a fact of course, but it can also be observed that free molecular oxygen is not thermodynamic stable on habitable planets. (As opposed to already made observations on hot Jupiters, if I remember correctly.)

Unless there is a biosphere supplying the oxygen, it will be sequestered in short order.

Well, I guess one scenario would be an advanced civilization moving their planet out to an acceptable orbit the more swollen their host star gets and then, should they survive the shedding of material during the transition to the white dwarf phase, they could conceivable move it back to a close orbit to maintain proper heat and light output. Of course that's a lof of assumptions and by that time they'd probably have left their system but who knows, the universe is a strange place.

With the technology levels you assume I would think it'd be far easier (and cheaper) just to terraform some other planet and move the population there rather than actually significantly shifting the orbit of a planet.

Possibly, you're probably right to a degree. Then again the energy potential lost abandoning their home planet versus moving it might make it a feasable option. It might make more sense to reinsert their home planet into an outer orbit either by itself or via a planet-moon arrangement with an outer gas giant (assuming our planetary model is common). Reminds me of the book A World Out Of Time.

Another limitation is that life as we know it here on our planet is the only kind of life we know how to look for. We have no understanding of what life would be like if it evolved under a completely different set of conditions.

This will be debated for long times to come, but one of the successes of the RNA world theory is that it now seems RNA is the only known molecule that, pre-proteinous enzymatically, could form replicators according to thermodynamics.

So it seems likely all life will start out with a simple RNA strand, lipid membrane universal common ancestor. (The simplest configuration.)

And for reasons of organic production and RNA thermodynamics it will first infect hydrothermal vent systems, where it maximizes the likelihood to capture redox metabolic processes.

So another not daring bet would be that the RNA UCA would evolve metabolism that can do what ours is doing. But of course with different enzymes. Divergence starts there.

A third not daring bet would be that the enzymes would soon be amino acid based. Similar to the simplest replicator configuration, the simplest known pathway to liberate RNA strand replicators from hot-cold vent cycle dependence is evolving some simple RNA strands to form "helper" molecules for strand separation, after their own separation. And then tie them to protein strand formation to make the simplest known molecular ratchet mechanism for cold separation.

This is the crucial step evolving from replication strands to genome hereditary mechanisms, and it seems the expansion of the biosphere from a localized to a global one can be tied to that.

g0m3r619 wrote:

There's a group of biologists who are trying to find alien life on our own planet.

I took a very dim view with these efforts, led by the deist Davies for his own reasons. (Seems it helps his magic views if life is not unique to Earth but, I would guess, if humans are.)

And then they instigated the "arsenic life" debacle. I rest my case. =D

If our Sun were to die and become a white dwarf, wouldn't the Earth be destroyed in the process? Seems looking for similar sized planets under these circumstances would be easier but pointless.

What the article didn't mention is that the people involved in the study are looking for systems with a white dwarf where the planet was caught around the white dwarf by either getting there after the death of the star, or being formed after the red giant phase. The process would be long enough (billions of years) for new life to evolve on the planet orbiting the white dwarf.

This is probably not the most common type of planetary system, but there are enough candidates out there to make it possible for us to spot such systems.

That key bit of information about looking for planets captured / formed after red giant phase is pretty critical to the understanding of the study being reported on. I had wondered about how likely a planet would stay in the Goldilocks zone before and after, then was reminded about the red giant phase. It is just too crucial bit of information to leave out of the article.

"Late at night, a police officer finds a drunk man crawling around on his hands and knees under a streetlight. The drunk man tells the officer he’s looking for his wallet. When the officer asks if he’s sure this is where he dropped the wallet, the man replies that he thinks he more likely dropped it across the street. "Then why are you looking over here?", the befuddled officer asks. Because the light’s better here, explains the drunk man."

A white dwarf is a very old star, so any planets still there are going to be old. It is supposed that silicon/oxygen heavy protostellar clouds will evolve planetary systems which will tend to concentrate free O2 in the atmospheres of rocky planets over very long spans of time. Mostly because of water dissociation and the escape of hydrogen from the planet. So an O2 signal may not be a sufficient condition of extant life.

Another problem is that present day white dwarf stars formed from metal poor clouds from the early universe. There may not *be* rocky planets.

hmm.. I thought most hints of life on earth will be destroyed when the Sun becomes a Red Giant... before it becomes a white dwarf....for instance water will evaporate, the atmosphere itself will be destroyed and most of earth would melt... when the Sun becomes a Red Giant. At least that was my understanding.

If that is that case, wouldn't the same happen to the Earth-like exo-planet orbiting a white dwarf? Would there be any useful chemical compounds left over...

Either way, any advances made in detecting exo planets is fascinating stuff.

jes

It would be. This is a terrible idea.

The only way to detect life around such a system would be to detect life on a planet which (almost certainly) came to be there AFTER the star went crazy, because the star would sterilize or destroy any life-carrying planet.

I mean, its a good idea to look at atmopsheres like this, but chances are you aren't going to be finding oxygen this way.

But you know, its easier to get funding if you at least pretend you're looking for life.

a white dwarf is much smaller than a star, so if a planet passes between it and us, far more light is blocked

If the radius of a white dwarf is about the same as the radius of the earth, it would be around 100 times smaller than the radius of the sun.As a result, the probability of observing a transit for a white dwarf is also 100 times smaller than for a typical yellow star.So, wouldn't this make it that much harder to look for (extinct) life around a white dwarf?

I would just like to clarify a couple points that many people seem to be assuming.

1. When a star goes through the process of becoming a white dwarf, at no time does it explode in a supernova-type fashion. First, the outer envelope of the star will expand into a Red Giant. In the case of our Sun, the radius will extend almost to the Earth's orbit. Sure, you lose Mercury and Venus and eventually Earth to tidal decay, but the outer solar system objects still exist: possibly Mars, asteroid belt, giant planets and their moons, comets, etc. Over time, the star's fusion-burning fuel will extinguish and the outer envelope of the star will expand and dissipate, leaving behind the white dwarf core.

2. Sure, throughout this period, you've probably eradicated the majority of living organisms (because you severely altered the habitable zone), but that's not to say that the possibility of creating new life has been extinguished. By the time our Sun gets to that point, we will have likely polluted most of the solar system with various biological materials, if not by directly going there ourselves, then by sending probes and satellites contaminated with bacteria or other single celled organisms. White dwarf's stay hot for billions of years and cool over time (which may produce complications for a stable habitable zone), giving plenty of time for new life to develop in the system.

The major consideration is that between the giant phase and white dwarf phase, the habitable zone has changed and you're probably stirring up the planetary orbital properties quite a bit. Based on the seemingly endless variety of planetary configurations that exist around stars, the probability that a stabilized White Dwarf system has a habitable planet is not so different than any given star.

In any case, it's a great way to mature technology and detection techniques before going out and doing the really hard stuff.

If our Sun were to die and become a white dwarf, wouldn't the Earth be destroyed in the process? Seems looking for similar sized planets under these circumstances would be easier but pointless.

What the article didn't mention is that the people involved in the study are looking for systems with a white dwarf where the planet was caught around the white dwarf by either getting there after the death of the star, or being formed after the red giant phase. The process would be long enough (billions of years) for new life to evolve on the planet orbiting the white dwarf.

This is probably not the most common type of planetary system, but there are enough candidates out there to make it possible for us to spot such systems.

That key bit of information about looking for planets captured / formed after red giant phase is pretty critical to the understanding of the study being reported on. I had wondered about how likely a planet would stay in the Goldilocks zone before and after, then was reminded about the red giant phase. It is just too crucial bit of information to leave out of the article.

Agreed. It was certainly mentioned in the original paper:

"A small planet within several AU cannot survive the asymptotic giant branch phase, and therefore needs to migrate in from a wider orbit to the habitable zone, after the WD has formed. Even then, Barnes & Heller (2012) and Nordhaus & Spiegel (2012) have emphasized that the tidal heating of the planet, until it had achieved full circularization and synchronization, would lead to full loss of any water and volatiles present. We note, however, that the young Earth was also a hot and dry place, but volatiles and water were then delivered to it by a barrage of comets. The comet impact rate then decreased to its present low level, greatly lowering the biological damage of such impacts. It is not implausible that such post-formation volatile delivery also could take place on an earth-like planet in a WD’s habitable zone, perhaps driven by the same scattering process that drove the planet itself to migrate inward after the formation of the WD."

a white dwarf is much smaller than a star, so if a planet passes between it and us, far more light is blocked

If the radius of a white dwarf is about the same as the radius of the earth, it would be around 100 times smaller than the radius of the sun.As a result, the probability of observing a transit for a white dwarf is also 100 times smaller than for a typical yellow star.So, wouldn't this make it that much harder to look for (extinct) life around a white dwarf?

Well, for older white dwarfs, they are relatively cool with a very small surface area, so the habitable zone is very close to the star (~0.01 AU). So, a planet that ended up close to the star via scattering mechanisms would be very easy to detect and fixes your factor of 100.

Ah... searching for life among the stars: The proverbial "needle in the haystack" endeavor.

So, some enterprising scientists figured it'd be easier to find that needle if they searched a smaller haystack. The problem being, they are also now searching for a smaller needle.

Except they aren't searching for a smaller needle, they're searching for a less-probable needle. They have to search a lot more haystacks, but it takes so much less time to search each one that they will find a needle faster than if they looked in big haystacks. Then there is the added bonus that the smaller haystack makes observing the interesting properties of the needle easier, once they find one.

a white dwarf is much smaller than a star, so if a planet passes between it and us, far more light is blocked

If the radius of a white dwarf is about the same as the radius of the earth, it would be around 100 times smaller than the radius of the sun.As a result, the probability of observing a transit for a white dwarf is also 100 times smaller than for a typical yellow star.So, wouldn't this make it that much harder to look for (extinct) life around a white dwarf?

Well, for older white dwarfs, they are relatively cool with a very small surface area, so the habitable zone is very close to the star (~0.01 AU). So, a planet that ended up close to the star via scattering mechanisms would be very easy to detect and fixes your factor of 100.